As semiconductor devices are highly integrated, a photlithographic technique for forming a fine resist pattern with high precision has been vigorously studied. For example, a technique capable of forming a pattern 0.3 micron or less wide on a substrate is required for a semiconductor element which operates at several tens of GHz. However, since the resist materials used as masks for etching are limited, light having a wavelength of 248 microns or longer has to be used. In this respect, the precision of a pattern is generally the same as the wavelength of light.
FIG. 9 is a view showing a reducing projection exposure apparatus which is generally used for such pattern formation. In the exposure apparatus, a wafer 15, on which a resist having a predetermined thickness is disposed, is put on a wafer stage 14 and light from an extra-high pressure mercury lamp as a light source passes through a photomask on which a predetermined mask pattern is formed, that is, a reticle 12 and then it is projected onto to the resist on the wafer 15 through a reducing projection lens 13. In addition, .theta..sub.1 designates an angle formed by the optical axis of the projection lens 13 and a line formed by connecting an intersecting point of the optical axis and the wafer to the radius of the exit pupil of the projection lens.
In general, the limit of resolution (R) of the reducing projection exposure apparatus is represented by the numerical aperture (NA=sin.theta..sub.1) of the projection lens and the wavelength (.gamma.) of light as shown in the following equation (1), where k.sub.1 is a constant indicating resist performance which is 0.612 in a case of a spherical projection lens according to a theory by Rayleigh. EQU R=k.sub.1 .gamma./NA (1)
In addition, when the resist pattern is actually formed, the depth of focus (DOF) shown in the following equation (2) is necessary for a curve or a step difference of the substrate. The DOF is generally set to 1.5 microns or more. In the equation (2), k.sub.2 is a constant. EQU DOF=k.sub.2 .gamma./(NA).sup.2 ( 2)
Meanwhile, it is thought that .gamma. may be decreased or NA may be increased according to the equation (1) in order to form a fine exposure pattern by the reducing projection exposure apparatus. However, when .gamma. is decreased, it becomes very difficult to make a lens and when the NA is increased, the DOF is reduced. Thus, there is a limit in forming the fine resist pattern in the exposure apparatus in addition to the resist material and it is difficult to actually form a pattern of at least the same resolution as the wavelength.
As described above, recently, a method for forming a resist pattern using a phase shift mask has been proposed as a methods of forming a fine resist pattern not restricted by the resist material and the exposure apparatus.
FIGS. 7(a) and 7(b) are views showing the structure and a principle of a phase shift mask, in which FIG. 7(a) is a sectional schematic view showing the structure of a phase shift mask and FIG. 7(b) is a graph showing a light intensity distribution on a surface perpendicular to an optical axis of a lens in a case where coherent light is partially applied to the phase shift mask shown in FIG. 7(a) and the diffracted light is focused by the lens. Referring to these figures, reference numeral 10 designates a phase shift mask which comprises a quartz substrate 2 and a phase shifter 1. In addition, in FIG. 7(b), the part where the light intensity is nearly 0 corresponds to an edge of the shifter 1.
Next, a method for forming a resist pattern using the phase shift mask will be described hereinafter.
The phase shift mask 10 is used as a reticle. When light from a light source is applied to a wafer on which a resist is applied by the reducing projection exposure apparatus shown in FIG. 9, the light from the light source is diffracted by an edge of the phase shifter 1 which comprises a wide resist towards a lower part of the phase shifter 1. Thus, a region in which a light intensity is lowered is only formed just under the edge of the phase shifter 1. Then, the transmitted light having the light intensity distribution shown in FIG. 7(b) is applied to the wafer on which the resist is present. Then, the resist on the wafer is subjected to pattern exposure and normal developing processing. As a result, a fine resist pattern having a precision which is one-half of the wavelength of the light can be formed. In addition, when a negative type resist is used as the resist on the wafer, an opening pattern is formed Just under the edge of the phase shift mask. Alternatively, when a positive type resist is used, the resist just under the edge of the phase shift mask is left on the wafer and a resist pattern in the form of a line is formed on the wafer.
FIGS. 8(a) and 8(b) are sectional views showing steps in manufacturing the phase shift mask. In these figures, the same references as those in FIGS. 7(a) and 7(b) designate the same or corresponding parts and reference numeral 1a designates a resist responsive to a negative type electron beam (referred to as an EB hereinafter) such as chloromethyl styrene. More specifically, as shown in FIG. 8(a), the resist 1a for the negative type EB is applied to the quartz substrate 2 and an EB exposure and developing process are normally performed. Then, as shown in FIG. 8(b), the resist 1a at an exposed region is left on the substrate 2 and then the phase shift mask 10 shown in FIG. 7(a) can be produced.
According to the conventional method for forming a resist pattern, the phase shift mask is used as a reticle and a fine exposure pattern is projected onto a resist and then the resist is developed. As a result, a fine resist pattern of a precision equal to or less than the wavelength of exposing light can be formed.
However, in the conventional resist pattern forming method using the phase shift mask, while a fine resist pattern of the same precision as the wavelength of light or less can be formed, an edge angle .theta..sub.2 of the phase shifter 1 varies in manufacturing the phase shift mask as shown in FIG. 8. Therefore, the light intensity distribution of the exposure pattern obtained through the phase shift mask 10 shown in FIG. 7(b) also varies, so that the contrast of the light intensity in the light intensity distribution can not be constant. Thus, in order to obtain a fine resist pattern having a predetermined width, the amount of development of the resist has to be controlled by changing the developing conditions, such as developing time each time in developing the resist after the exposure pattern is applied. However, since this process is very troublesome and can not be performed with high precision, the dimensions of the resist pattern still vary, so that it is very difficult to form the resist pattern having a predetermined width easily with high yield and reproducibility. Especially, when the contrast of the light intensity in the exposure pattern is reduced, the reactivity of the resist in the developing process is reduced. Therefore, if the amount of development of the resist is increased by increasing the developing time, the difference in the development of an upper layer and a lower layer of the resist is increased. For example, when an opening pattern is formed on the negative type resist, the opening becomes overhanging shape and the dimensional precision of the pattern is reduced. When such a resist pattern is used in the processing of the wafer thereafter, the processing dimensions vary and characteristics of the product can be adversely affected.